Vehicular antenna system



Nov. 20, 1951 w. w. SCHWEISS ET AL 2,575,471

VEHICULAR ANTENNA SYSTEM Filed April 13, 1950 2 SHEETSSHEET l Nov. 20, 1 1 w. w. scHwElss ET AL VEHICULAR ANTENNA SYSTEM 2 SHEETS-SHEET 2 Filed April 15, 1950 Patented Nov. 20, 1951 VEHICULAR ANTENNA SYSTEM Walter W. Schweiss and Raymond I. Gratzncr,

Philadelphia, Pa., assignors to Philco Corporation, Philadelphia, Pa,

' sylvania a corporation of Penn- Application April 13, 1950, Serial No. 155,608

The invention herein described and claimed relates to an antenna system for mobile Very-highfrequency radio apparatus.

In accordance with the present invention, the metal body of the motor vehicle or other mobile craft is utilized as the antenna, and novel means are provided for coupling to the metal-body antenna.

While the invention may be utilized for transmitting as well as receiving, it will be convenient to describe the invention in connection with the reception, rather than the transmission, of veryhigh-frequency radio waves.

The invention requires that the surface of the metal body, which is to function as the receiving antenna, include a discontinuity of substantial length, i. e. a length equivalent to at least onehalf wavelength at the frequencies to be received. While the discontinuity may comprise a long narrow strip of insulating material separating two surfaces of the metal vehicle-body, the discontinuity preferably comprises a long narrow airgap or slot in the body surface. It appears that the currents, which are induced in the metal body of the vehicle by the radio waves which impinge thereagainst, produce an electromagnetic field in the air gap or slot and that the potential differences between spaced-apart points along the edges of the slot, particularly between points spaced one-quarter wavelength apart, are substantially greater than the potential differences between points on the vehicle body correspondingly remote but surrounded by large metal areas. We have discovered that by coupling to the slot in a manner or manners to be described, signal energy of suflicient magnitude is derived for satisfactory application to the inputcircuits of the receiver. It is believed important for the coupling means to effect a reasonably good impedance match between the metal-body antenna and the transmission line leading to the receiver.

We have tested the invention extensively in motor vehicles, particularly in the reception of frequency-modulation broadcast signals in the 88-108 megacycles band and, to a lesser extent in the reception of television signals in the 54-87 and 174-216 megacycles bands.

While a special slot or other discontinuity may be provided in the surface or skin of the motorvehicle body, we have discovered that very satisfactory results may be obtained by using a slot already available in most conventional motor-vehicle bodies. Particularly good results are obtained by coupling to the trunk slot, 1. e., to the slot defined by the edges of the trunk lid and 8 Claims. (01. 250-33) trunk portion of the car body. These edges are ordinarily spaced from each other by a small dimension, of the order of one-quarter of an inch. Alternatively, coupling may be made to the motor slot, i. e. to the slot located between'the motor hood and the car body. This is less desirable, however, than coupling to the trunk slot since motor ignition'interference is encountered. As another alternative, coupling may be made to a door slot, i. e., to the slot between a 'dooredge and the edge of the door frame. This also is less desirable than coupling to'the trunk slot, the installation being more difficult and the results not being quite as good. f

In View of the above, and in order to simplify the remaining portion of this specification, we will assume, in the detailed description which follows, that coupling is made to the'trunk slot.

As indicated previously, it appears that the slot to which coupling is to be made should have an effective length equal to'at least one-half wavelength at the frequencies to be received. For example, one-half wavelength in'free space at the lowest frequency of the frequency-modulation band (88 mo.) is about 67 inches, but the physical length of one-half wavelength (at 88 me.) along the trunk slot of the metal vehicle body is sub: stantially less than the said '67 inches since the propagation factor is substantially less than unity. In most present day motor cars, the physical length of the trunk slot from the trunk lock to the nearest hinge substantially exceeds onehalf wavelength at 88 megacycles and the trunk slot between these points is, therefore, suitable for use in connection with the reception of FM signals. The trunk slot between these points is also suitable for use in the reception of television signals since the physical length thereof, in most motor cars, is equal to-at least one-half wavelength at the lowest frequency presently employed in television transmission. If desired, the length of the trunk slot may be determined or controlled by metallic strips speciallyprovided at desired points and so mounted on the trunk portion as to make conductive contact between the'trunk body and the lid when the lid is in closed position.

The coupling means employed to'couple to the trunk slot may be conveniently disposed along, and supported by, the 'U-shaped flange or channel which serves to support the trunk lid. While it may be possible to employ other means of coupling to the trunk slot, we have discovered that the best results arefobtainedvby employing, as the coupling means, a two-conductor stub, either of coaxial cable or twin-lead, having a length equivalent to at least one-quarter wavelength. In the preferred embodiment, a one-half wavelength stub is employed, connected in a manner which will be described in detail.

We have found that the preferred embodiment of the new antenna system (comprising the metal vehicle-body antenna, the trunk slot; the" stub coupling to the trunk slot, and the transmission line leading to the receiver), functions in an effective and superior manner. The radiation pattern is cardioid or heart-shaped, i. e., reception is excellent from all directions except that in which the motor vehicle facing. Th'e null point, however, is of such narrow width as not to constitute a serious impairment. It will be recognized, for example, that the cardioid-pattern is substantially better than the well-known figure-eight pattern of a dipole antenna.

'I he theoretical explanation of the manner in which the new antenna system functions is quite complex and not 'completelyunderstood. We believe,'however, that the-stub functions, not only as "a neans for couplingto the trunkslot and to the fields established in the-trunk slot, but also as an impedance-matching device, matching to a satisfactory degree the impedancesof the-vehiclebody antenna and the transmission line leading to the receiver. I I I It is the broad object then of this invention to provide an inexpensive, easily installed, antenna system for a mobile very-high-frequency radio apparatus, utilizing' the metal body of the motor vehicle, or other mobile craft, as an antenna.

I A more specific object'of this invention is to provide simple, readily installed; means for coupling the metal-body antenna to the transmission line leading to the'radio apparatus.

I These and other objects. features and advantages of the present invention, and the manner which the objectsare attained, will be best understood from a consideration of the following detailed description of a preferred embodiment taken together with the accompanying drawings wherein: I I I I I I I F ig'ur'e 1 shows a side 'el ev ati onal view of a; motorvehicle in which 'theantenna system of the present invention has been installed, in preferred mannenin the trunk channel; I

Figure 2'shows'the rearelevational view of the same motor vehicle as is shown in Figure 1;

Figure 3 shows schematically the preferred manner in which the half-wavelength coaxial stub is disposed in the trunkchannel and connected to the metal body of the'vehicle;

I Figure 4 is a sectional view'showing the preferred manner of placing the coaxial stub'within the special gasket which replaces the conventional trunk gasket; and

I Figures 5 and 6 show schematically alternative stub arrangements, Figure 5 showing a half-wave parallel-lead stub, and Figure 6 showinga quarter-wave coaxial stub, in the trunkchannel.

Referring now to Figures 1 and 2, there is shown a motor vehicle 40 having at the rear a conventional trunk portion H and trunk lid l2. A radio receiver is shown mounted beneath the instrument panel. Extending from receiver IE to the trunk portion II is a transmission line [B which is desirably a shielded line, preferably a coaxial cable. The transmission line It preferably has' a length equivalent to an integral number of one-half wavelengths at the center frequency of the band of' frequencies to be received so that the impedance looking toward the receiver l5 from the trunk end of the transmission line may be substantially equivalent to the input impedance of the receiver.

Transmission line It terminates at a point i! on the trunk channel 18 (see Figure 4) which serves to support the trunk lid l2 when in closed position. In the illustrated embodiment, transmission line it is shown terminating on the channel l8 near the lower right-hand corner of the trunk; but the transmission line may, if desired, terminate elsewhere, as for example, near the lower left-hand corner, or near either the upper rightor left-hand corners.

At point ll, transmission line It connects to an insulated two-conductor stub 20 which extends along the trunk channel I8 for a distance which, in the preferred embodiment, is equivalent to about one-half wavelength at the center frequency of the band of frequencies to be received. The velocity of wave propagation along the stub is substantially less than unity and the'physical length of one-half wavelength of stub issubstam tially shorter than one-half wavelength in free space. In practice, We have found the propagation factor to be of the order of0.6.

It will be understood that where stub 20- and transmissionline it each comprises, as is prefer-- able, a length of coegcial cable, the stub merely constitutes a continuation of the coaxial transmission line. The conductors of the coaxial stub may be insulated from the trunk portion H and lid l2 by substituting, for the conventionalgasket ordinarily found along the trunk channel, a gasket 23 (Figure 4) having a centrally-positioned longitudinal hole, and placing the coaxial stub 26 therein, as shown in Figure '4. II

Figures 3 and 4 show schematically the connections which we have found to produce the best results. At point H, where transmission line it terminates and stub 20 begins, the outer conductor of the coaxial line 16 is connected to the'trunk portion ll, preferably to the edge of side wall {9 of trunk channel l8, as shown in Figure 4'. At the far end 2| of the half-wavelength coaxial stub 20, the inner and outer conductors of the stub are shorted together. And, at point 22, located one-quarter wavelength back from the shorted end 2i, the inner conductor of the coaxial stub is connected to the trunk portion ll, preferably to edge of side wall 19 of trunk channel It. It will be apparent that since stub 20 is one=half wavelength long and the distance between points 21 and 22 is one-quarter wavelength, the distance between points 22 and I1 is also one-quarter wavelength. I I I I While we prefer that the half-waveshort'ed s'tubbeaco'axial stub as shown in' Figures '3 and 4, it may be a parallel-lead stub, as shown schematically in Figure 5. The'co'n'nections of the parallel-lead stub 41] are similar to those described above with respect to the coaxial stub 20, i. e. at a point 42 located one-quarter wavelength back from the shorted end M, "the stub conductor which connects to the inner'conductor of the coaxial transmission line 16* isconnected to 'the trunk portion ll, preferably to the edge of side wall IQ of trunk channel 18. And'at point 31, where transmission line 16 terminatesand stub 49 begins, the outer conductor of the coaxial transmission line is connected to trunk portion H, preferably to *the edge of sidewall l9 of channel 18. The results obtained frbrrr the ar rangement shown in Figure 5 arealmostas good as that obtained from'the preferred-arrangement [6 shown inFigure 3.

Somewhat less satisfactory results are obtained from the arrangement shown in Figure 6. In Figure 6, a quarter-wave shorted coaxial stub 50 is placed in trunk channel l8 and, at a point 52 one-quarter wavelength from the shorted end the inner conductor is connected to the trunk portion ll, preferably to the edge of the side wall l9. At a point located one-half wavelength from shorted end 5| and hence outside of the channel 18, a connection is made from the outer conductor-of coaxial transmission line 16 to a point on the trunk portion ll, preferably to the side wall I9. Alternatively, as in the case of the halfwave stub, the quarter-wave stub 50 may be comprised of parallel-lead instead of coaxial cable.

As indicated previously hereinabove, we are not certain of the theoretical explanation why the arrangements shown in the drawing and described hereinabove function so satisfactorily. However, we do know from repeated tests and measurements that the antenna system shown and described outperforms the standardvertical or horizontal rod or dipole type of antenna. The undesirable so-called picket-fence effect resulting from phase changes between direct and reflected signals, is substantially entirely eliminated, probably due to the fact that the motor-vehiclebody antenna exceeds a wavelength, both in length and width. And, as previously indicated, the radiation pattern is cardioid, which constitutes an improvement over the figure-eight pattern of a dipole.

While we are uncertain as to the theory of operation of the arrangements described, the following comments which are based upon the results observed, may be of interest.

(a) It appears that the potential differences between nodal and anti-nodal points along wall [9. of the channel [8, particularly along the edge of wall l9, are of substantial magnitude and considerably greater than the potential differences between nodal and anti-nodal points located on the surface of the vehicle body and surrounded by large areas of metal.

(1)) In the arrangement shown in Figure 3,

point I! is a node and point 22 is an anti-node. At point [1, the outer conductor of the coaxial line is held substantially at ground potential while the potential at point 22, one-quarter wavelength away, varies in accordance with-the signal wave impinging upon the vehicle-body antenna.

(0) The potential variations across points I! and 22 on the channel wall I9 are effectively applied between the inner and outer conductors of the coaxial stub 20 by the connection of the inner conductor to the channel wall at point 22.

(d) The quarter-wave shorted stub (between points 22 and 2|) is believed to function as an impedance-matching device, matching to a satisfactory degree, the impedance of the vehiclebody antenna and of the transmission line I6. In most of our tests, we employed a low-impedance (SO-ohm) coaxial transmission line. However, the impedance of the vehicle-body antenna is probably substantially lower than 50 ohms.

(e) It appears that the quarter-wave shorted coaxial stub (between points 22 and 2|) derives energy inductively from the electromagnetic field present in channel [8. This field, which appears to be of considerable strength, is believed to be attributable to the long narrow air-gap or slot 4). The air-gap or slot is narrowest at 24 and I4 and the field is probably of greatest strength at these points. However, the field within the channel I8 at the location of the coaxial stub is also of-considerable strength. The outer conductor ofthe coaxial cable functions as an electrostatic shield, and, at high frequencies, also as a magnetic shield, at least to a certain extent. It appears, however, that the outer conductor is not completely efieotive as ,a magnetic shield at the frequencies with which we are concerned.

For, it appears that voltages are induced within the coaxial stub by reason of its location within the magnetic field. In any event, we have observed that if the quarter-wave shorted stub (between points 22 and 2| be taken out of the channel 18 (the connections, including the con- 'nection at point 22, remainin as shown in Figure -3-)--the signal wave applied to the receiver is substantially-reduced. -We believe, therefore, that energy is transferred from the vehicle-body antenna to the transmission line I6 conductively (by connection 22) and also inductively.

Y (1) The connection of the outer conductor of the coaxial line to the trunk channel at point I! tends substantially to eliminate standing waves from the transmission line Hi. We have observed that the-standing-wave ratio on line I6 is reduced very substantially when the connection at H is made. v

' (g) It appears that the quarter-wave length of stub between'points l1 and 22 also derives energy inductively from the electromagnetic field of the channel, for if the stub (between points I! and 22) be removed from the trunk channel l8, a reduction in the amount of signal energy applied to the receiver i5 is observed. However, the reduction is substantially less than that sustained when the quarter-wave shorted stub (between points 22 and 2|) is removed from the channel.

Accordingly, we have shown, in Figure .6, an

alternative arrangement wherein only the quarter-waveshorted stub (between points 52 and 5|) is contained within the channel l8. The quarterwave-section-between points 52 and 4'! is outside the channel. 1

It willbe understood that in each of the arrangem'ents shown (Figures 3, 5 and 6), the shortwhich exists between the trunk lid l2 and the vehicle body ll, particularly between the trunk lid and the trunk channel I8 at 24 and I4 (Figure apparatus is used broadly and is intended to include, for example, television, facsimile, and teletype apparatus.

Having described our invention, we claim:

1. In combination; a mobile craft having a conductive body; a radio apparatus mounted in said craft; a long narrow slot in said body, said slot having a physical length equivalent to at least one-half wavelength at the center frequency of a band of carrier frequencies to be received; a half-wave shorted stub disposed along said slot; a low-impedance connection between one conductor of said stub and one edge of said slot at a point approximately one-quarter wavelength from said shorted end; a low-impedance connection between the other conductor of said stub and said edge of said slot at the open end of said stub; and a transmission line connecting said open end of said stub to said apparatus.

2. In combination; a mobile craft having a conductive body; a radio apparatus mounted in said craft; a long narrow slot in said body, said slot having a physical length equivalent to at least one-half wavelength at the center froceived; a halnwave:shortednoaxialistubzdisposed.

along said slot; .a low impedance;connectionvbetween theeinner conductor; of saidsstub-androne edge of said slot ata point approximately one quarter wavelength from said shorted rend; allowimpedance connection between theouten-com ductor of said stub and saidedge.ofzsaidxslotat 4. In combination; amotor vehicle-having; a:

conductive :body, including. a: trunk; portion havingan openingv therein and a-.lid-therefor-, said trunk. portion including, aroundsthe-peripheryi of said: opening, a channeled. flange for: supporting.

said lid. when in closed position, saidilidv and trunk portion being insulated from: each other except at spaced points; a radio apparatus mounted in said motor vehicle'and adapted. to operate in a very-high-frequency; band; and means for utilizing said motr-vehicle-:-body as an antenna. for said apparatus, ,said .means including; coupling means. comprising a. half-wave shorted stub disposed along aportion. of said channeled flange between two of said points. of connection spaced at least one-half, wavelength apartat the center frequencyof said'band; alow impedance connection between one conductor. 01 said stub and said trunk portion ata pointaape proximately one-quarter wavelength from said shorted end; alow impedance.connectionbetween the other. conductor ofsaid stub andsaid trunk portion at the :open end of .said stub; ,and atransa mission line connecting said open endof said-stub to said apparatus.

5. In combination; a motor vehicie havinga:

conductive body including a trunk portion having an opening therein and a lidtherefor saidtrunk portionincluding, around the periphery of. said opening,. a channeled flange for supportingsaid lid when in closed position, said lid and trunkv means: comprising: a". hali jwave shorted. coaxial stub (disposed along a; portion of said channeled fiangezbietween twoof said pointsof connection spaced vatileastone-half wavelength. apart; a low impedance; connection between the-inner; COR? ductor; of said: stuband saidwtrunk portioniat: a point'approximatelyone-quarterwavelength from saidnshortedend; alow impedanceconnection be.- tweenzthe; outer conductor. of said: stubvand, said trunkiportionlat the-open end of saidstubuand accoaxialtransmission line connecting said open end: ofsaidcstub .to:said apparatus.

6-. The combinationv claimed, in. claim 5: char: acterizedsby the fact that said coaxial. transmis-. sion: line: has. a length equivalent to" an integral number ofhalf-wavelengtha.

'1; In: combination; a. mobile craft:v having; a. conductive; body; aradio apparatus; mounted; in said .tcraft, said apparatus :being adapted to .openateiini a1.very-highefrequencycband; a slottin said body'havinga physical length equivalentto. at leastone-half wavelength at thecenter frequency ofzsaidz-band; a quarter-wave shorted coaxiaistub disposcdalong saidslot; a low-impedance. con: nection between. the inner conductor ofsaid: stub and one edge of said slot at the open endof said stub; and-a coaxiaitransmission line connecting said'openend ofsaid stub to' said apparatus, the outer. conductor of said coaxialtransmissionline being connected to .said body. at a point lone-half wavelength. from the shorted end of said stub.v

8. The combination claimed in claim '7. characterized'by the fact that between said apparatus andsaid point where said outerconductor isconnected. to said bodysaid transmission: line hasna length equivalent" to an integral number of half wavelengths.

WALTER W. SCHWEISS. RAYMOND I. GRATZNER.

REFERENCES. CITED The following references are of record in the file of this patent:

UNITED STATESv PATENTS Number Namev Date,

2,115,764 Carter May 3, 1938 2,129,852 Leib Sept. 13,1938 2,238,438. Alford Apr. 15, 1941 2,513,097" Darling J une. 27. 1950 2,517,968 Brown. Aug. 8, 19.50 

